In a process called tidal heating, gravitational push and pull from Jupiter’s Galilean moons — Europa, Ganymede, Io and Callisto — and the gas giant itself stretch and squish the moons enough to warm them. As a result, some of the icy moons contain interiors warm enough to host oceans of liquid water, and in the case of the rocky moon Io, tidal heating melts rock into magma. Planetary researchers previously believed that Jupiter was responsible for most of the tidal heating associated with the liquid interiors of the moons, but Dr. Hamish Hay of NASA’s Jet Propulsion Laboratory and colleagues found that moon-moon interactions may be more responsible for the heating than Jupiter alone.
“Maintaining subsurface oceans against freezing over geological times requires a fine balance between internal heating and heat loss, and yet we have several pieces of evidence that Europa, Ganymede, Callisto and other moons should be ocean worlds,” said co-author Dr. Antony Trinh, a postdoctoral researcher in the Lunar and Planetary Laboratory at the University of Arizona.
“Io, the moon closest to Jupiter, shows widespread volcanic activity, another consequence of tidal heating, but at a higher intensity likely experienced by other terrestrial planets, like Earth, in their early history.”
“Ultimately, we want to understand the source of all this heat, both for its influence on the evolution and habitability of the many worlds across the solar system and beyond.”
“It’s surprising because the moons are so much smaller than Jupiter,” Dr. Hay said.
“You wouldn’t expect them to be able to create such a large tidal response.”
The trick to tidal heating is a phenomenon called tidal resonance.
“Resonance creates loads more heating. Basically, if you push any object or system and let go, it will wobble at its own natural frequency,” Dr. Hay explained.
“If you keep on pushing the system at the right frequency, those oscillations get bigger and bigger, just like when you’re pushing a swing.”
“If you push the swing at the right time, it goes higher, but get the timing wrong and the swing’s motion is dampened.”
Each moon’s natural frequency depends on the depth of its ocean.
“These tidal resonances were known before this work, but only known for tides due to Jupiter, which can only create this resonance effect if the ocean is really thin (less than 300 m, or under 1,000 feet), which is unlikely,” Dr. Hay said.
“When tidal forces act on a global ocean, it creates a tidal wave on the surface that ends up propagating around the equator with a certain frequency, or period.”
According to the team’s model, Jupiter’s influence alone can’t create tides with the right frequency to resonate with the moons because the moons’ oceans are thought to be too thick.
It’s only when the researchers added in the gravitational influence of the other moons that they started to see tidal forces approaching the natural frequencies of the moons.
When the tides generated by other objects in Jupiter’s moon system match each moon’s own resonant frequency, the moon begins to experience more heating than that due to tides raised by Jupiter alone, and in the most extreme cases, this could result in the melting of ice or rock internally.
For moons to experience tidal resonance, their oceans must be tens to hundreds of kilometers thick, which is in range of scientists’ current estimates. However, there are some caveats to the new findings.
“Our model assumes that tidal resonances never get too extreme,” Dr. Hay said.
“We want to return to this variable in the model and see what happens when they lift that constraint.”
“We also are hoping that future studies will be able to infer the true depth of the oceans within these moons.”
The findings were published in the journal Geophysical Research Letters.
Hamish C.F.C. Hay et al. Powering the Galilean Satellites with Moon-Moon Tides. Geophysical Research Letters, published online July 19, 2020; doi: 10.1029/2020GL088317